Method for the production of liquefied natural gas and nitrogen

ABSTRACT

A method for the production of liquefied natural gas (“LNG”) and nitrogen is provided. The method may include the steps of: a) providing a nitrogen production facility, wherein nitrogen production facility comprises: a main heat exchanger, an air separation unit, a nitrogen recycle compressor, a first nitrogen refrigeration supply configured to provide refrigeration to the main heat exchanger for cooling a main air feed, b) providing a secondary refrigeration supply; c) liquefying a natural gas stream using refrigeration from the secondary refrigeration supply to form an LNG product stream; wherein the secondary refrigeration supply is configured to compress and expand a refrigerant to produce refrigeration, wherein the refrigerant of the secondary refrigeration supply is shared with refrigerant of the first nitrogen refrigeration supply

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser. No. 62/201,947 filed on Aug. 6, 2015, U.S. Provisional Application Ser. No. 62/305,381 filed on Mar. 8, 2016, and U.S. Provisional Application Ser. No. 62/370,953 filed on Aug. 4, 2016, all of which are hereby incorporated by reference in their entireties.

TECHNICAL FIELD OF THE INVENTION

The present invention generally relates to a method and apparatus for efficiently producing liquefied natural gas (“LNG”).

BACKGROUND OF THE INVENTION

Small scale nitrogen generators (e.g., typically 100 mtd to 400 mtd) are commonly needed at remote locations where the cost of an air separation unit producing O₂ and N₂ is not required or justified. The nitrogen can be (1) low purity gaseous for utility purposes (e.g., ethane cracker), (2) liquid form for fracking purposes, or (3) higher purity merchant product, particularly in locations such as North Dakota or Alberta. These sites can also have a demand for small quantities of liquefied natural gas, which is often trucked in at a high price. Alternatively, small scale LNG plants could be installed but also result in a high cost of product, thereby making these options uneconomical.

Therefore, it would be advantageous to provide a method and apparatus that could produce gaseous nitrogen, and/or LIN, while also producing LNG in a more efficient manner.

SUMMARY OF THE INVENTION

The present invention is directed to a method and apparatus that satisfies at least one of these needs. In certain embodiments, the invention can provide a lower cost, more efficient and flexible method to produce LNG. For example, in certain embodiments, the invention can include integration of a nitrogen production facility with an LNG production facility.

In one embodiment, refrigeration energy from the nitrogen production unit can be partially diverted to be used to liquefy natural gas. In one embodiment, the incoming air and natural gas are cooled in the same heat exchanger. In another embodiment, the liquefaction of the natural gas occurs in a separate heat exchanger. In embodiments in which the liquefaction of natural gas occurs in a second heat exchanger, the invention can also include an additional set of turbine boosters configured to provide additional refrigeration for the LNG heat exchanger. In one embodiment, the discharge pressure of the turbine for the LNG heat exchanger is approximate the column reboiler condensing pressure.

In one embodiment, a method for the production of liquefied natural gas (“LNG”) and nitrogen is provided. In this embodiment, the method can include the steps of: a) providing a nitrogen production facility, wherein nitrogen production facility comprises: a main heat exchanger, an air separation unit, a nitrogen recycle compressor, a first nitrogen refrigeration supply configured to provide refrigeration to the main heat exchanger for cooling a main air feed; b) providing a secondary refrigeration supply; c) liquefying a natural gas stream using refrigeration from the secondary refrigeration supply to form an LNG product stream; wherein the secondary refrigeration supply is configured to compress and expand a refrigerant to produce refrigeration, wherein the refrigerant of the secondary refrigeration supply is shared with refrigerant of the first nitrogen refrigeration supply.

In optional embodiments of the method for the production of LNG and nitrogen:

-   -   the secondary refrigeration supply comprises a third         turbine-booster, wherein the third turbine-booster has a third         booster and a third turbine;     -   the natural gas stream is liquefied in a secondary heat         exchanger;     -   the main heat exchanger is configured to receive a main air feed         comprising purified and compressed air at a pressure of at least         3 bar, wherein the air separation unit is in fluid communication         with a cool side of the main heat exchanger, the air separation         unit being configured to receive cooled air from the main heat         exchanger and produce gaseous nitrogen and waste oxygen, wherein         the air separation unit comprises a single column having a         bottom reboiler and a top condenser, wherein the nitrogen         recycle compressor is in fluid communication with a warm side of         the main heat exchanger such that the nitrogen recycle         compressor is configured to receive a nitrogen recycle from the         main heat exchanger, wherein at least a portion of the nitrogen         recycle is made up of gaseous nitrogen from the air separation         unit, wherein the first nitrogen refrigeration supply comprises         a first turbine-booster which has a first booster and a first         turbine, the first booster in fluid communication with the         recycle compressor such that the first booster is configured to         receive a compressed nitrogen recycle from the recycle         compressor, and wherein the second nitrogen refrigeration supply         comprises a second turbine-booster which has a second booster         and a second turbine, the second booster in fluid communication         with the first booster such that the second booster is         configured to receive a boosted nitrogen from the first booster,         wherein an outlet of the second booster is in fluid         communication with the heat exchanger such that the boosted         nitrogen from the second booster is cooled within the heat         exchanger, wherein the second turbine is in fluid communication         with the heat exchanger such that the second turbine is         configured to receive a cooled fluid under pressure from the         heat exchanger and then expand the cooled fluid to provide         refrigeration for the apparatus;     -   the third booster is in fluid communication with the outlet of         the second booster such that the third booster is configured to         receive a portion of the boosted nitrogen from the outlet of the         second booster;     -   the method further includes the step of compressing the portion         of the boosted nitrogen in the third booster, cooling the         portion of the boosted nitrogen in the secondary heat exchanger,         expanding the portion of the boosted nitrogen to an expanded         pressure P_(E) to form a cold refrigerant, and warming the cold         refrigerant in the secondary heat exchanger against the natural         gas stream to form a warm refrigerant stream;     -   the expanded pressure P_(E) is selected based on a first         condition within the air separation unit;     -   the first condition includes a pressure of a reboiling fluid         configured to provide reboiling duty for the bottom reboiler of         the single column;     -   the first condition includes the vaporization temperature of         bottom liquids surrounding the bottom reboiler of the single         column;     -   the method further includes the step of cooling the warm         refrigerant stream in the main heat exchanger to form a cooled         refrigerant stream;     -   the cooled refrigerant stream, or a portion derived therefrom,         is used to provide reboiling duty to the bottom reboiler of the         single column;     -   the method further includes the steps of withdrawing a fraction         of partially compressed nitrogen recycle from an internal stage         of the nitrogen recycle compressor; introducing the partially         compressed nitrogen recycle to the main heat exchanger for         cooling before using the partially compressed nitrogen recycle         as the heating fluid for the bottom reboiler; and then flashing         the partially compressed nitrogen recycle into a top portion of         the single column;     -   the warm refrigerant stream is combined with the partially         compressed nitrogen recycle from the internal stage of the         nitrogen recycle compressor prior to cooling in the main heat         exchanger;     -   the third booster is in fluid communication with the outlet of         the first booster such that the third booster is configured to         receive a portion of the boosted nitrogen from the outlet of the         first booster;     -   the third booster is in fluid communication with the outlet of         the nitrogen recycle compressor such that the third booster is         configured to receive a portion of the boosted nitrogen from the         outlet of the nitrogen recycle compressor;     -   both the first and the second boosters are in fluid         communication with the outlet of the nitrogen recycle compressor         such that both the first and second boosters are configured to         receive a portion of the boosted nitrogen from the outlet of the         nitrogen recycle compressor;     -   the second turbine and third turbine are the same turbine;     -   the turbine outlet pressure is thermally linked with the         reboiler of the single column;     -   the method further includes the step of providing a third         refrigeration supply, wherein the third refrigeration supply         comprises a natural gas expansion turbine; and/or     -   the method further includes the step of expanding a high         pressure natural gas stream within the natural gas expansion         turbine to produce an expanded natural gas stream, and warming         the expanded natural gas stream in a secondary heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments.

FIG. 1 provides an embodiment of the present invention.

FIG. 2 provides an embodiment of the present invention.

DETAILED DESCRIPTION

While the invention will be described in connection with several embodiments, it will be understood that it is not intended to limit the invention to those embodiments. On the contrary, it is intended to cover all the alternatives, modifications and equivalence as may be included within the spirit and scope of the invention defined by the appended claims.

In one embodiment, the apparatus can include a Main Air Compressor (MAC), an air purification system, heat exchange means, a distillation column, and a nitrogen refrigeration cycle having a nitrogen recycle compressor and warm and cold turbine boosters. In one embodiment, the nitrogen refrigeration cycle provides compression energy for pressurized gaseous nitrogen product as well as refrigeration for both cooling down the incoming air and liquefy nitrogen, as well as providing refrigeration to liquefy the natural gas to LNG. In one embodiment, the warm turbine booster flow can be split to provide refrigeration to both the warm section of the nitrogen portion and the warm section of the LNG portion. In one embodiment, this warm turbine load can serve to supply two refrigeration loads, since both the nitrogen and LNG portion heat exchange cooling curves have similar pressures and temperatures in this warm section.

In one embodiment, the temperature at the cold end of the nitrogen portion of the heat exchanger can be about −178° C., and the cold end of the LNG portion is about −164° C. In one embodiment, different expanded nitrogen streams at different pressures and coming from different turbines are used to liquefy the LNG as compared to cool the incoming air and liquefy nitrogen. This is preferably done since the temperature differences and cooling curve differences make it less efficient to use the same turbine for both nitrogen and LNG exchangers. Also, if the separate cold turbine for the LNG portion is simply warmed up and uses the same pressure profile (for integration in to suction of the same Recycle compressor) then the process is not optimized.

In one embodiment, warming the cold end of the LNG exchanger is achieved by reducing the turbine pressure ratio. In one embodiment, the turbine discharge pressure can be increased to ˜10 bar to match the pressure for condensing in the column reboiler and the intermediate stage of the recycle compressor. This significantly reduces the flow rate at the first stage of the recycle compressor, and also improves the efficiency of the natural gas liquefaction, therefore both reducing power and cost.

In one embodiment, the method can include integrating a natural gas letdown system with a closed loop nitrogen refrigeration cycle. In this embodiment, the natural gas letdown essentially provides “free” refrigeration energy since the natural gas would have been alternatively letdown across a valve (i.e., the resulting drop in temperature of the natural gas would have been absorbed by the surroundings and would not have been recovered in any meaningful way). With the addition of a natural gas turbine booster and dryer (i.e., purification unit), LNG can be co-produced with a significant power savings, while also reducing the size of the nitrogen refrigeration cycle.

Referring to FIG. 1, a process flow diagram of an embodiment of the current invention is shown. In FIG. 1, main air feed 2, which has already been purified and compressed to a pressure of about 5 to about 6 bar, is introduced to heat exchanger 10 and cooled down to a temperature near its dew point or lower to form fully cooled air feed 12. Fully cooled air feed 12 is then introduced to air separation unit 19, in order to separate the various components of air. Waste gaseous oxygen 22 is recovered from air separation unit 19 and is passed through the cold side of heat exchanger 10 in order to provide cooling to heat exchanger 10. After exiting heat exchanger 10, waste gaseous oxygen 22 can be vented to the atmosphere, used to regenerate the air adsorbers (not shown) or sent to a system of columns (not shown) if recovery of the oxygen is desired.

Gaseous nitrogen 28 is also withdrawn from air separation unit 19 and passed through the cold side of heat exchanger 10 to provide additional cooling. However, instead of venting to the atmosphere, gaseous nitrogen 28 can be recycled in the process. Nitrogen recycle 38 exits main heat exchanger 10 and is introduced to recycle compressor 40 and compressed to form compressed nitrogen recycle 46. In one embodiment, if gaseous nitrogen is desired, a portion of compressed nitrogen recycle 46 can be removed and collected as gaseous nitrogen product 47. Compressed nitrogen recycle 46 is then cooled in second aftercooler 43 before being boosted in first booster 50 and cooled in third aftercooler 51 to form boosted nitrogen 52. Boosted nitrogen 52 is then introduced to second booster 53 in order to further compress boosted nitrogen 52 before being cooled in fourth aftercooler 55 to form fully boosted nitrogen 56. In one embodiment fully boosted nitrogen 56 can be at ambient temperature and a pressure of about 45 to about 65 bar prior to entering heat exchanger 10.

Fully boosted nitrogen 56 is then introduced to heat exchanger 10 for cooling. In one embodiment, one portion of fully boosted nitrogen 56 is fully cooled in heat exchanger 10 to form liquefied nitrogen 58, which is subsequently introduced to liquid/gas separator 60 by flashing via valve 59. In another embodiment, another portion of fully boosted nitrogen 56 is only partially cooled in heat exchanger 10 to form partially cooled boosted nitrogen 78. In one embodiment, partially cooled boosted nitrogen 78 is at or above its super critical pressure. Partially cooled boosted nitrogen 78 is then introduced into second turbine 80 in order to expand partially cooled boosted nitrogen 78 to form second expanded nitrogen 82. In one embodiment, second expanded nitrogen 82 can have a temperature that is near or below its dew point and a pressure of about 5 to about 6 bar. In one embodiment, second expanded nitrogen 82 is a two phase fluid consisting of gas and liquid phases. In a preferred embodiment, second expanded nitrogen 82 is introduced to liquid/gas separator 60 in order to separate any gaseous nitrogen from liquid nitrogen. Recovered liquid nitrogen 62 is withdrawn from liquid/gas separator 60 and collected as product. In one embodiment, gaseous nitrogen 68 is withdrawn from a top portion of liquid/gas separator 60 and combined with gaseous nitrogen 28 before introduction to the cold side of heat exchanger 10 and subsequently recycled.

In one embodiment, fraction of compressed nitrogen recycle 48 is withdrawn from compressed nitrogen recycle 46 and fed to the warm end of heat exchanger 10, where fraction of compressed nitrogen recycle 48 is partially cooled before being expanded in first turbine 70 to form first expanded nitrogen 72. In one embodiment, first expanded nitrogen 72 is reintroduced to heat exchanger 10, preferably at an intermediate point, and combined with gaseous nitrogen 28 and subsequently recycled. In one embodiment, first turbine 70 is connected by a common shaft with first booster 50 and helps to provide the energy needed for first booster 50 to compress compressed nitrogen recycle 46. Likewise, second turbine 80 is connected by a common shaft with second booster 53 and helps to provide the energy needed for second booster 53 to compress boosted nitrogen 52. In one embodiment, first turbine 70 and second turbine 80 provide substantially all of the refrigeration needs for the process.

First turbine 70 and second turbine 80 produce refrigeration by work expansion. Their respective boosters, first booster 50 and second booster 53, utilize the produced work to further compress their respective nitrogen streams.

Fraction of partially compressed nitrogen recycle 44 can be withdrawn from partially compressed nitrogen recycle 42 and, along with third expanded nitrogen 182 is fed to the warm end of heat exchanger 10. After exiting the cold end of heat exchanger 10, fraction of partially compressed nitrogen recycle 44 is used to provide heat to bottom boiler 21 before being introduced via valve 93 near a top portion of single column 20 (see FIG. 2). Those of ordinary skill in the art will recognize that even though recycle compressor 40 and second recycle compressor 45 are pictured as two different compressors, it is possible to use one compressor and remove fraction of partially compressed nitrogen recycle 44 from an inner stage of that single compressor. In another embodiment, the two sections of the nitrogen recycle compressor can be combined on a common shaft with the main air compressor (not shown).

In the embodiment shown, portion of fully boosted nitrogen 57 is compressed in third booster 153, cooled in fifth aftercooler 155 to produce supplemental compressed nitrogen 156, which is then partially cooled in LNG heat exchanger 120 to form partially cooled nitrogen 178. Partially cooled nitrogen 178 is then expanded in third turbine 180 to form third expanded nitrogen 182, which is then warmed in LNG heat exchanger 120 against natural gas in order to form LNG. Production of LNG can be controlled by LNG control valve 122. After warming, third expanded nitrogen 182 is then combined with fraction of partially compressed nitrogen recycle 44, which is then used as a reboiling fluid within the distillation column (see FIG. 2). Additional refrigeration for LNG heat exchanger 120 can be provided by removing a fraction of first expanded nitrogen 72 and then warming fraction of first expanded nitrogen 172 within LNG heat exchanger 120. After warming, fraction of first expanded nitrogen 172 is then combined with gaseous nitrogen 28 to form nitrogen recycle 38 and then recycled back to recycle compressor 40.

In one embodiment, stream 57 can also be derived from boosted nitrogen 52 or from stream 46, depending on the splits of power from turbines 70, 80 and 180, since these turbines set the pressure ratios of the boosted nitrogen streams. In one embodiment, it is preferred to have the pressures of streams 56 and 156 be elevated, while at the limit of the design pressure of secondary exchanger.

Regarding FIG. 2, a simplified process flow diagram showing the interconnectivity of the LNG refrigeration cycle with the reboiler of the distillation column is provided. It is important to note here that FIG. 2 is a simplified version of the process flow diagram, in which many of the process streams have been removed for easier viewing. For a more detailed look at all of the components of the nitrogen production unit, attention should be drawn to U.S. Pub. 2015/0168057 and U.S. Pat. Pub. 2015/0168058, both of which are herein incorporated by reference in their entireties.

In FIG. 2, gaseous nitrogen near the top of single column 20 travels up tube 27, with a portion being withdrawn as gaseous nitrogen 28 and the rest condensing within top condenser 23 before being reintroduced to single column 20. Oxygen-rich condensing fluid introduced near top condenser 23 provides the needed cooling to condense the nitrogen. Waste gaseous oxygen 22 is withdrawn and used to provide refrigeration to heat exchanger 10

The vapor from the top of condenser 23 is the waste stream of the air distillation process and can be withdrawn through the plant equipment and sent to atmospheric vent. The pressure at the condenser 23 can be 1.3-1.5 bara. This pressure and the composition of this waste stream yields a vaporizing temperature of about −179° C. in condenser 23 which is thermally linked to the condensing N₂ of stream 27, resulting in a pressure in column 20 (which is approximately 5 bara). Reboiler 21 thermally links the vaporization of the oxygen rich liquid with a condensing nitrogen stream. This vaporizer operates by condensing nitrogen at approximately −170° C., thereby setting the pressure of the condensing nitrogen at approximately 9.3 bara.

Matching the discharge pressure of the third turbine 180 to the pressure of the reboiler can help allow the LNG section and N₂ section to operate more efficiently (i.e., that is with a specific power equal to that of separate independent liquefiers, as well as a significant capex savings (shared recycle compressor and warm turbine booster)). As mentioned before, if the third expanded nitrogen pressure is at a higher level than the suction pressure of the nitrogen recycle compressor, the heat exchange efficiency of the natural gas liquefaction process can be improved. Operating at the ˜9 bara (or ˜8 to 11 bar range) allows the LNG liquefier to operate at its optimum point. But this third expanded nitrogen pressure is limited by the possible condensation of nitrogen at the turbine discharge, facilitated at higher pressure

The term “ambient temperature” if used herein refers to the temperature of the air surrounding an object. Typically the outdoor ambient temperature is generally between about 0 to 110° F. (−18 to 43° C.).

The term “cryogenic gas” if used herein refers to a substance which is normally a gas at ambient temperature that can be converted to a liquid by pressure and/or cooling. A cryogenic gas typically has a boiling point of equal to or less than about −130° F. (−90° C.) at atmospheric pressure.

The terms “liquefied natural gas” or “LNG” as used herein refers to natural gas that is reduced to a liquefied state at or near atmospheric pressure.

The term “natural gas” as used herein refers to raw natural gas or treated natural gas. Raw natural gas is primarily comprised of light hydrocarbons such as methane, ethane, propane, butanes, pentanes, hexanes and impurities like benzene, but may also contain small amounts of non-hydrocarbon impurities, such as nitrogen, hydrogen sulfide, carbon dioxide, and traces of helium, carbonyl sulfide, various mercaptans or water. Treated natural gas is primarily comprised of methane and ethane, but may also contain small percentages of heavier hydrocarbons, such as propane, butanes and pentanes, as well as small percentages of nitrogen and carbon dioxide.

While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.

“Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.

“Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited. 

We claim:
 1. A method for the production of liquefied natural gas (“LNG”) and nitrogen, the method comprising the steps of: a) providing a nitrogen production facility, wherein nitrogen production facility comprises: a main heat exchanger, an air separation unit, a nitrogen recycle compressor, a first nitrogen refrigeration supply configured to provide refrigeration to the main heat exchanger for cooling a main air feed; b) providing a secondary refrigeration supply; c) liquefying a natural gas stream using refrigeration from the secondary refrigeration supply to form an LNG product stream; wherein the secondary refrigeration supply is configured to compress and expand a refrigerant to produce refrigeration, wherein the refrigerant of the secondary refrigeration supply is shared with refrigerant of the first nitrogen refrigeration supply.
 2. The method as claimed in claim 1, wherein the secondary refrigeration supply comprises a third turbine-booster, wherein the third turbine-booster has a third booster and a third turbine.
 3. The method as claimed in claim 2, wherein the natural gas stream is liquefied in a secondary heat exchanger.
 4. The method as claimed in claim 3, wherein the main heat exchanger is configured to receive a main air feed comprising purified and compressed air at a pressure of at least 3 bar, wherein the air separation unit is in fluid communication with a cool side of the main heat exchanger, the air separation unit being configured to receive cooled air from the main heat exchanger and produce gaseous nitrogen and waste oxygen, wherein the air separation unit comprises a single column having a bottom reboiler and a top condenser, wherein the nitrogen recycle compressor is in fluid communication with a warm side of the main heat exchanger such that the nitrogen recycle compressor is configured to receive a nitrogen recycle from the main heat exchanger, wherein at least a portion of the nitrogen recycle is made up of gaseous nitrogen from the air separation unit, wherein the first nitrogen refrigeration supply comprises a first turbine-booster which has a first booster and a first turbine, the first booster in fluid communication with the recycle compressor such that the first booster is configured to receive a compressed nitrogen recycle from the recycle compressor, and wherein the second nitrogen refrigeration supply comprises a second turbine-booster which has a second booster and a second turbine, the second booster in fluid communication with the first booster such that the second booster is configured to receive a boosted nitrogen from the first booster, wherein an outlet of the second booster is in fluid communication with the heat exchanger such that the boosted nitrogen from the second booster is cooled within the heat exchanger, wherein the second turbine is in fluid communication with the heat exchanger such that the second turbine is configured to receive a cooled fluid under pressure from the heat exchanger and then expand the cooled fluid to provide refrigeration for the apparatus.
 5. The method as claimed in claim 4, wherein the third booster is in fluid communication with the outlet of the second booster such that the third booster is configured to receive a portion of the boosted nitrogen from the outlet of the second booster.
 6. The method as claimed in claim 4, further comprising the step of compressing the portion of the boosted nitrogen in the third booster, cooling the portion of the boosted nitrogen in the secondary heat exchanger, expanding the portion of the boosted nitrogen to an expanded pressure P_(E) to form a cold refrigerant, and warming the cold refrigerant in the secondary heat exchanger against the natural gas stream to form a warm refrigerant stream.
 7. The method as claimed in claim 6, wherein the expanded pressure P_(E) is selected based on a first condition within the air separation unit.
 8. The method as claimed in claim 7, wherein the first condition includes a pressure of a reboiling fluid configured to provide reboiling duty for the bottom reboiler of the single column.
 9. The method as claimed in claim 7, wherein the first condition includes the vaporization temperature of bottom liquids surrounding the bottom reboiler of the single column.
 10. The method as claimed in claim 6, further comprising the step of cooling the warm refrigerant stream in the main heat exchanger to form a cooled refrigerant stream.
 11. The method as claimed in claim 10, wherein the cooled refrigerant stream, or a portion derived therefrom, is used to provide reboiling duty to the bottom reboiler of the single column.
 12. The method as claimed in claim 11, further comprising the steps of withdrawing a fraction of partially compressed nitrogen recycle from an internal stage of the nitrogen recycle compressor; introducing the partially compressed nitrogen recycle to the main heat exchanger for cooling before using the partially compressed nitrogen recycle as the heating fluid for the bottom reboiler; and then flashing the partially compressed nitrogen recycle into a top portion of the single column.
 13. The method as claimed in claim 12, wherein the warm refrigerant stream is combined with the partially compressed nitrogen recycle from the internal stage of the nitrogen recycle compressor prior to cooling in the main heat exchanger.
 14. The method as claimed in claim 4, wherein the third booster is in fluid communication with the outlet of the first booster such that the third booster is configured to receive a portion of the boosted nitrogen from the outlet of the first booster.
 15. The method as claimed in claim 4, wherein the third booster is in fluid communication with the outlet of the nitrogen recycle compressor such that the third booster is configured to receive a portion of the boosted nitrogen from the outlet of the nitrogen recycle compressor.
 16. The method as claimed in claim 4, wherein both the first and the second boosters are in fluid communication with the outlet of the nitrogen recycle compressor such that both the first and second boosters are configured to receive a portion of the boosted nitrogen from the outlet of the nitrogen recycle compressor.
 17. The method as claimed in claim 4, wherein the second turbine and third turbine are the same turbine.
 18. The method as claimed in claim 17, wherein the turbine outlet pressure is thermally linked with the reboiler of the single column.
 19. The method as claimed in claim 1, further comprising the step of providing a third refrigeration supply, wherein the third refrigeration supply comprises a natural gas expansion turbine.
 20. The method as claimed in claim 19, further comprising the step of expanding a high pressure natural gas stream within the natural gas expansion turbine to produce an expanded natural gas stream, and warming the expanded natural gas stream in a secondary heat exchanger. 